The primary roof support systems used in coal mines include headed rebar bolts typically 4 feet to 6 feet in length, ¾ inch and ⅝ inch in diameter, and used in conjunction with resin grouting in 1 inch diameter holes.
Typically, grouting is accomplished using multi-compartment resin cartridges. For example, as disclosed in U.S. Pat. No. 3,861,522 to Llewellyn, a compartmented package may be used in which a longitudinal diaphragm, made of film, is sealed to the inner wall of a circular tubular member, also made of film, to form at least two continuous longitudinal linear junctures therebetween in a manner such that the volumes of the resulting compartments are variable. A two-compartment package having infinite relative volume variability in both compartments is preferred.
Another resin capsule for mining roof bolting systems is disclosed in U.S. Pat. No. 4,239,105 to Gilbert. A multi-compartment capsule has a longitudinally extending tubular inner compartment for containing a catalyst and a longitudinally extending tubular outer compartment around the inner compartment for containing a resin composition. The longitudinally extending tubular inner and outer compartments are formed by a single-ply film strip of polyester material to which are welded along opposite longitudinal edges two-ply strips having a first ply of polyester material welded to the polyester film strip of the single-ply strip and a second ply of polyethylene material. The composite sheet made up of the single-ply strip and two-ply strips is folded upon itself along one two-ply marginal edge to form the tubular inner compartment so that the polyethylene ply material at the composite strip edge contacts such ply material when folded and is welded thereto. The opposite two-ply marginal edge of the composite strip is then folded over the formed inner compartment so as to bring the polyethylene ply material at the edge into contact with the polyethylene ply material of the other edge and is welded thereto to form the tubular outer compartment.
Resin cartridges can suffer from a substantial stiffness problem. In particular, as disclosed in U.S. Pat. No. 6,545,068 B1 to Simmons et al., within about two months, the cartridges become limp or lacking in stiffness or firmness such that a cartridge containing a grouting composition is not as rigid as the cartridge originally packaged. Limp cartridges are a problem for the customer because limp cartridges are difficult to insert into overhead boreholes. A process is disclosed for substantially improving the firmness of a grouting composition, with a compressible substance introduced into the grouting composition. The compressible substance is selected from the group consisting of nitrogen, oxygen, air, carbon dioxide, foam rubber, carbon dioxide, argon, and combinations of two or more thereof.
Turning to the mine bolts used in combination with the resin cartridges, ¾ inch bolts offer significant performance advantages over ⅝ inch bolts. First, as compared to the use of ⅝ inch bolts, when ¾ inch bolts are installed in 1 inch diameter holes, a smaller annulus is provided between the bolt and the borehole wall thereby allowing improved mixing of resin and catalyst in the annulus. The improved mixing especially is important with resin cartridges that have a high ratio of resin to catalyst. Such resin cartridges typically have resin/catalyst ratios of between about 60/40 and about 90/10. Second, the resin cartridges typically employed when grouting bolts in holes must be “shredded” in situ to release the resin and catalyst in the cartridge and then permit mixing thereof. Generally, it is known that when there is an annulus larger than ⅛ inch, large pieces of the shredded cartridge can significantly interfere with resin/catalyst mixing and concomitantly result in poor strength between the bolt and borehole wall. This phenomenon of “glove fingering” is known to occur when the plastic film that forms the cartridge lodges in the borehole proximate the surrounding rock, thereby interrupting the mechanical interlock desirably formed by the resin and rock. Third, the smaller annulus provided by a ¾ inch bolt in a 1 inch hole (e.g., an annulus of ⅛ inch) provides a much stiffer support system, with less movement per unit of load and less creep. Such strength advantages have been demonstrated in extensive studies conducted in Australia and the United States, and in fact show that a 1/16 inch annulus is even better. See, e.g., C. Mark, C. Compton, D. Oyler and D. Dolinar, “Anchorage pull testing for fully grouted bolts,” Paper #13 in 21st International Conference on Ground Control in Mining, Aug. 6-8, 2002, ISBN 0-939084-56-9.
Despite its disadvantages, the ⅝ inch bolt is used in approximately 50% of coal mines because it is lighter, easier to use, and lower in cost than the ¾ inch bolt. Lower weight bolts are advantageous in the mine setting. When a bolt is inserted for example in a 1 inch diameter hole that is 4 feet to 6 feet in length, the gap between the bolt and hole must completely be filled with resin to ensure sufficient strength in grouting. By using a smaller diameter bolt and more resin, the overall weight of the grouting system is lowered because the specific gravity of the bolt is about 8.6 as compared to about 2.0 for the resin. The difference in weight between a ⅝ inch bolt and a ¾ inch bolt fully grouted with resin in a 4 foot long hole is about 1 pound per bolt. Although this weight disparity may be accommodated by drilling a smaller diameter hole thus requiring less resin, a hole with a diameter smaller than 1 inch is more difficult and expensive in mining operations because of limitations associated with removal of the drill cuttings and the stiffness of typical drill bits.
Waste polyethylene terephthalate (PET) polymer is neither environmentally biodegradable nor compostable, and thus suffers from disposal problems. Recycling has become a viable alternative to the long-term accumulation of garbage, and there is substantial availability of recyclable PET waste. For example, post-consumer PET derived from soft drink bottles is available in bulk for relatively low cost. Bottle-grade PET is known to be of high quality among the various PET grades, and is available particularly as washed flakes. There are major incentives from environmental perspectives and governmental regulatory directives to find new and improved uses for PET waste.
British Published Patent Application GB 2 138 732 A relates to an anchor useful in mining, construction and civil engineering. The published patent application discloses a sleeve formed on the end of an anchor element, e.g., rebar, by placing an end of the element in a reusable plastics mould, having a former for at least one thread, and containing a self setting shrinking composition. According to the patent application, the composition may be cementitious or resinous: polyester or epoxy resin compositions. As described, a quantity of polyester grout Lokset is supplied into the mould to cast a sleeve on a threaded bar. The published patent application, however, does not discuss the use of PET, the desirability of using post-consumer recycled PET, or the use of injection molded polymer coatings on mine supports.
British Patent Specification 1,382,054 is directed to fixing bolts in blind holes and discloses a method of securing a bolt in a hole comprising applying a sleeve to an end portion of the bolt. According to the specification, the sleeve may be formed of wood or a hardenable composition, e.g., a filled resin or a cementitious mortar. Also, the sleeve may be preformed of a plastics material, and the specification states that suitable plastics materials for such sleeves are: polyacetal or polyformaldehyde resin, polyamides e.g. nylon acrylonitrile/butadiene/styrene copolymers, polyolefins or any other polymer having the requisite or desired properties. The specification further discloses that one end of a bolt may be inserted in a mold and a castable composition poured in to form the sleeve; the composition sets and any shrinkage takes place in making the sleeve. The published patent specification, however, does not discuss the use of PET, the desirability of using post-consumer recycled PET, or the use of injection molded polymer coatings on mine supports.
Despite these developments, there remains a need for a mine support that is formed in part of post-consumer recycled polymers such as PET. There further remains a need for mine supports with polymer coatings formed of materials by methods other than casting, such as injection molding. Also, there remains a need for mine supports that are formed in part of thermoplastic polymer coatings that are molded onto the supports.
As an unreinforced, semi-crystalline thermo-plastic polyester, PET has excellent wear resistance, excellent hardness and stiffness, very good creep resistance, low coefficient of friction, high flexural modulus, and superior dimensional stability. Nevertheless, PET—particularly following melting and resolidifying of post-consumer recyclable waste—can be quite brittle and as such can be inappropriate in some applications.
U.S. Pat. No. 6,583,217 B1 to Li et al. is directed to a composite material composed of fly ash and waste PET. Melted waste, chemically unmodified PET material and fly ash particles are mixed in a vessel to disperse fly ash particles in the melted PET material. The resulting mixture then is cooled to solidify the melted PET material to form a composite material having a matrix comprising PET and dispersoids distributed in the matrix and comprising fly ash particles.
There exists a need for a mining bolt system that overcomes the disadvantages of poor mixing and poor shredding of the resin capsule package. In particular, there exists a need for a composite mining bolt system that will support a load of ten tons with less than ½ inch deflection in a 12 inch anchored length.
In addition, despite known composite materials, there exists a need for alternative PET materials with less susceptibility to brittle behavior. More particularly, there exists a need for a PET material appropriate for use in coating metals used in mechanically rigorous applications. Also, there exists a need for a composite material formed of cement and recycled PET, along with a method of making same.